The present invention relates to a system for controlling the rotational speed of AC motors, in particular capacitor motors or shaded-pole motors, in which case a controllable electronic switching device which is connected upstream of the motor is actuated by a control unit such that a sinusoidal input AC voltage is used to produce a motor AC voltage which can be varied in order to change the rotation speed.
As a rule, capacitor or shaded-pole motors are operated from a single-phase AC power supply and are used, for example, for driving fans, pumps or the like. In the case of such drives, for example in the case of fans, it is frequently necessary for various power levels (airflow levels) to be available, normally in specific steps, for different operating modes. Thus, for example in the case of fume extraction hoods, various fan rotation speeds may be used in order to vary (increase/reduce) the airflow. It is also often necessary to adjust the rotation speed, particular in steps, in air-conditioning systems, heat exchangers or, in an entirely general manner, in the case of fan and pump drives (flow machines).
At relatively low power levels, it is generally normal for this purpose to connect series resistors of different magnitudes in the current path to the motor via a stepping switch, in order in this way to achieve a reduction in the motor voltage, and thus to change the rotation speed. However, this method is very uneconomic since the voltage dropped on the resistor causes a power loss, that is to say is converted into heat, which in many cases also still has to be dissipated.
DE 42 22 431 Al discloses a stepped winding switching system for single-phase electric motors, in which it is possible to switch between a plurality of auxiliary windings by means of a plurality of triacs or thyristors, in order to vary the rotation speed. However, such winding switching is complex since winding starts and winding ends must be routed to the exterior in order to connect them. Furthermore, a separate switching element (triac, thyristor) must be provided for each switching step.
DD 94 666 discloses a system of the generic type mentioned above. This document specifically describes a method for setting the rotation speed of brushless induction motors, in particular of asynchronous squirrel-cage rotor motors. In this case, a switching device in the form of two back-to-back connected thyristors is connected upstream of the motor, or of the motor winding, and these are driven by a control device such that entire half-cycles or full-cycles of the power supply voltage are respectively switched off or switched on (passed through). This allows the frequency of the motor voltage to be varied. However, it has been found that such actuation results in relatively poor efficiency.
Similar methods are in each case disclosed in DD 216 586 and DE 28 42 391 Al, although these relate specifically to applications for three-phase or polyphase motors. In this case as well, however, entire cycles of the power supply voltage are in each case switched off or passed through as the motor voltage.
DE 38 30 196 Al discloses a method for using phase-gating controllers as frequency converters. In this case as well, entire cycles of the power supply voltage are in each case switched off by always presetting a correspondingly large, unchanging (constant) gating angle. This is thus, actually, not a phase-gating controller in the conventional sense, since the individual cycles are always either passed through completely or not at all, but are never actually only partially gated. Cyclically changing between half cycles which are switched on and switched off results in a voltage profile whose fundamental frequency is the desired, wanted frequency.
On the other hand, DE 34 27 479 Al has now also disclosed a method for gating control of voltage half cycles of a three-phase controller having two current paths (which can be controlled in parallel but in opposition) per phase, for a squirrel-cage rotor motor. In this case, in order to operate three-phase motors at stepped operating rotation speeds, positive and/or negative half-cycles are gated in accordance with various patterns in such a manner that a frequency which differs from the power supply frequency is achieved in order to vary the rotation speed in an appropriate manner. However, this method relates exclusively to three-phase motors and not to single phase AC motors, such as capacitor motors or shaded pole motors.
All these known control systems have the main disadvantage of relatively poor efficiency. In some cases, disturbing noise also occurs.
The present invention is based on the object of providing a control system of this generic type, by means of which optimum, low-noise motor operation is achieved, in particular with improved efficiency and an improved torque profile, over a wide rotation speed setting range and, preferably, largely independently of the load. In this case, it is also intended to be possible to achieve the control method using technically simple and cost effective means.
This is achieved according to the invention in that the control unit is designed in such a manner that the fundamental frequency and/or the amplitude of the motor AC voltage can be varied by phase gating, with the individual half-cycles having periodically recurring triggering angles. The period length in this case governs the fundamental frequency. It is thus possible, according to the invention, to use the input AC voltage normally from the power supply voltage to generate virtually any desired number of motor AC voltages with different fundamental frequencies and/or waveforms, thus making it possible to achieve motor operation that is always optimum, in particular with good efficiency and a high starting torque.
The fundamental frequency can be varied in a manner known per se by the control unit switching of specific half cycles or full cycles of the sinusoidal input AC voltage in order to form voltage gaps, and passing through other specific cycles. According to the invention, phase gating of successive voltage half cycles with "asymmetric" phase-gating angles (triggering angles) which are the same but, in particular, differ, can be carried out in combination with this. This is dynamic phase-gating control. In a particularly advantageous refinement for example for the 25 Hz fundamental frequency the invention in this case provides that, in the case of a full cycle which follows a voltage gap, the phase-gating angle of the first half-cycle is greater than the phase-gating angle of the subsequent, second half-cycle, to be precise in particular with a ratio such that a motor current resulting from the motor AC voltage produced in this way has a profile which is essentially symmetrical with respect to the zero line. This measure according to the invention is based an the knowledge that, in the case of known methods in which entire cycles are in each case switched off and voltage gaps are produced in this way, the cycle which occurs after a voltage gap initially leads to a relatively steep current rise, to be precise owing to the fact that there is still not back-emf in the motor winding at this time. However, after this, the current then decreases as the emf caused by the flux builds up, thus resulting in an asymmetric current profile relative to the zero line; the current then has a DC voltage element since it is shifted relative to the zero line, which also, in consequence, leads to poor efficiency. This asymmetry of the motor current is advantageously compensated for by the dynamic phase-gating control according to the invention, so that a motor current profile can be achieved which is symmetrical with respect to the zero line and has no DC voltage element. In this way, the efficiency is improved, and the energy consumption is minimized.